Semiconductor integrated circuit

ABSTRACT

A semiconductor integrated circuit including: a circuit block having an internal voltage line; an annular rail line forming a closed annular line around the circuit block and supplied with one of a power supply voltage and a reference voltage; and a plurality of switch blocks arranged around the circuit block along the annular rail line, the plurality of switch blocks each including a voltage line segment forming a part of the annular rail line and a switch for controlling connection and disconnection between the voltage line segment and the internal voltage line.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2007-241517 filed in the Japan Patent Office on Sep. 18,2007, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor integrated circuit thatcontrols, by a switch, connection and disconnection between the internalvoltage line of a circuit block and wiring to which a power supplyvoltage or a reference voltage is applied.

2. Description of the Related Art

MTCMOS (Multi-Threshold Complementary Metal Oxide Semiconductor)technology is known as technology for controlling the shutoff of powersupply to a circuit and the cancellation of the shutoff by a switch.

Generally, the threshold voltage of a transistor in a logic circuit orthe like needs to be lowered as a design value in order to prevent asignal delay accompanying a decrease in power supply voltage or anelement miniaturization. When the threshold voltage of a transistor in alogic circuit or the like is low, a high leakage current occurs. TheMTCMOS technology prevents unnecessary consumption of power by a circuitin a stopped state by shutting off a leakage current path of the circuitin the stopped state by means of a transistor (power supply switch)having a higher threshold value than a transistor in a logic circuit orthe like.

In application of the MTCMOS technology to a circuit block, internalvoltage lines referred to as a so-called virtual VDD line and aso-called virtual GND line are provided within the circuit block. Theinternal voltage lines are connected to a global real power supply line(real VDD line) and a real reference voltage line (real VSS line) forestablishing connection between blocks outside the circuit block, via apower supply switch for power shutoff and the cancellation of theshutoff.

The power supply switch is provided at three kinds of positions, thatis, a position between a functional circuit that is started and stoppedrepeatedly and the real VDD line, a position between the functionalcircuit and the real VSS line, and both the positions. In general, aPMOS transistor is used as the switch on the VDD side, and an NMOStransistor is used as the switch on the VSS line side.

The starting and stopping of the functional circuit in the MTCMOSapplied block is controlled by a circuit in an MTCMOS non-applied blockset in an operating state at all times while supplied with power fromthe real VDD line and the real VSS line after the semiconductorintegrated circuit is started. Alternatively, a configuration can beadopted in which a control signal for controlling the starting andstopping of the functional circuit in the MTCMOS applied block can beinput from an external terminal of the semiconductor integrated circuit.

A power supply switch can be realized by a cell within an MTCMOS appliedblock. More specifically, there is a case where within an MTCMOS appliedblock, a power supply switch is provided in each logic circuit cell ofan inverter, a NAND circuit, a NOR circuit or the like, or in afunctional circuit cell realized by a few logic circuits, and there is acase where a dedicated power supply switch cell without a logic circuitor a functional circuit is provided. A switch arrangement of this typewill hereinafter be referred to as an “internal switch (SW)arrangement,” and a semiconductor integrated circuit adopting thearrangement will hereinafter be referred to as an “internal SWarrangement type IC.”

As opposed to the internal SW arrangement type IC, a semiconductorintegrated circuit in which power supply switches are arranged around acircuit block as an object of power supply control is known (seeJapanese Patent Laid-Open No. 2003-289245, hereinafter referred to asPatent Document 1 and Japanese Patent Laid-Open No. 2003-158189,hereinafter referred to as Patent Document 2, for example). A switcharrangement of this type will hereinafter be referred to as an “externalswitch (SW) arrangement,” and a semiconductor integrated circuitadopting the arrangement will hereinafter be referred to as an “externalSW arrangement type IC.”

The external SW arrangement is suitably used in combination with acircuit block having a general-purpose circuit (for example a memory, aCPU or the like) referred to as a so-called “macro” as a part or thewhole of the circuit block.

Patent Document 2 discloses a configuration in which a transistor cell(switch) is disposed on three sides or four sides of a circuit block,the switches having a shape such that the direction of length of theswitches is along the respective sides and the direction of arrangementof a transistor gate line within the switches is the same as thedirection of the length.

In this configuration, a VDD supply ring and a VSS supply ring arearranged as an annular line annularly enclosing the periphery of thecircuit block on an opposite side (outer side) of a transistor cellarrangement region from the circuit block. The VDD supply ring and thedrain of a switch transistor are connected to each other by a metalwiring layer at a level different from that of the VDD supply ring. TheVSS supply ring and a VSS line within the circuit block are connected toeach other by a metal wiring layer at a level different from that of theVSS supply ring. The source of the switch transistor and a virtual VSSline within the circuit block are connected to each other by a metalwiring layer at a level different from that of the VSS supply ring.

SUMMARY OF THE INVENTION

A semiconductor integrated circuit described in the above PatentDocument 1 does not have a VDD voltage supply line disposed in the formof a ring around a circuit block (internal circuit). A voltage droptherefore occurs at a position of the VDD voltage supply line whichposition is distant from a VDD voltage supply source. As a result, evenwhen a switch transistor of a same size as other switch transistors isturned on or off for a same time, the switch transistor has a differentcapability to charge or discharge an internal voltage line of theinternal circuit. A device is thus necessary such that the size of theswitch transistor is increased or the number of transistors is increasedas the position of the switch transistor becomes more distant from theVDD voltage supply source. This results in a disadvantage of an increasein leakage current via the switch transistor in a state in which theinternal circuit is stopped.

The threshold voltage of the switch transistor is set higher than thethreshold voltage of a transistor in the internal circuit, so that theleakage current is relatively small. However, when a stop period islong, unnecessary power consumption due to an increase in the size ofthe switch transistor or the number of switch transistors may not beignored.

The semiconductor integrated circuit described in the above PatentDocument 2 has a VDD voltage supply line and a VSS voltage supply linearranged in annular form as a supply ring. The semiconductor integratedcircuit described in the above Patent Document 2 therefore uniformizes avoltage drop from a voltage supply source to each switch transistor ascompared with the semiconductor integrated circuit described in theabove Patent Document 1 which circuit does not have the supply ring.Thus, the increase in leakage current is relatively suppressed.

However, the semiconductor integrated circuit described in the abovePatent Document 2 has the annular lines (supply rings) and switchtransistors arranged separately from each other, and thus has complexwiring for connecting the switch transistors and the supply rings (theVDD supply ring and the VSS supply ring) to each other.

While the VDD supply ring and the VSS supply ring are provided for apurpose of uniformizing voltage drops, the potentials of the VDD supplyring and the VSS supply ring may not be perfectly uniform as a result ofbeing affected by an operating peripheral circuit. Of course, thepotentials become more uniform by reducing the wiring resistance ofthese supply rings. However, the potential of the VSS supply ring, forexample, in the vicinity of a peripheral circuit frequently operatingwith a high amplitude may be raised from a reference potential (forexample 0 [V]) as a result of being affected by the peripheral circuit.It is difficult, however, to perfectly estimate the operation of theperipheral circuit. Therefore, in order to avoid the effect of theperipheral circuit and stabilize the operation of the switchtransistors, a need to shift the position of a switch transistor orchange the size of a switch transistor arises in a process of design,and a heavy design load of performing connection wiring again isimposed.

That is, the configuration of the switch transistor (referred to as a“macro” in Patent Document 2) described in Patent Document 2 does nothave a structure ready for such a design change.

Incidentally, various examples of modification disclosed in PatentDocument 2 in which examples the size of the switch transistor ischanged or the switch transistor is doubly provided also clearlyindicate the importance of the position and the size of the switchtransistor.

A semiconductor integrated circuit according to a first embodiment ofthe present invention includes: a circuit block having an internalvoltage line; an annular rail line forming a closed annular line aroundthe circuit block and supplied with one of a power supply voltage and areference voltage; and a plurality of switch blocks arranged around thecircuit block along the annular rail line, the plurality of switchblocks each including a voltage line segment forming a part of theannular rail line and a switch for controlling connection anddisconnection between the voltage line segment and the internal voltageline.

In the present invention, preferably, the plurality of switch blockshave the voltage line segment and the switch internally connected toeach other, and positional relation of end sides of the voltage linesegment at two opposed sides of a block frame is set identical in theplurality of switch blocks.

In the present invention, preferably, the plurality of switch blocks arestandardized by having an identical size and including four differentkinds of switch blocks for four respective sides of the circuit block.

Alternatively, preferably, for the plurality of switch blocks, a numberof switch blocks arranged on each side of the circuit block isdetermined such that the number of switch blocks is increased asimpedance of wiring supplying one of the power supply voltage and thereference voltage to the circuit block as viewed from each of the foursides of the circuit block is decreased.

In the present invention, preferably, a wiring part of the annular railline which wiring part is parallel with one pair of opposed sides offour sides of the circuit block and a wiring part of the annular railline which wiring part is parallel with another pair of opposed sides ofthe four sides of the circuit block are formed by wiring layers atdifferent levels.

In the present invention, preferably, in each of the switch blocks, thevoltage line segment is disposed in one of states of intersecting andbeing in proximity to an arrangement region of the switch, a buffercircuit operating receiving voltage supplied from the voltage linesegment is formed on an opposite side of the arrangement region of theswitch and the voltage line segment from the circuit block, a firstcontrol line segment to which a control signal is input via a controlline is disposed on an input side of the buffer circuit, and a secondcontrol line segment electrically connected to a control node of theswitch is disposed on an output side of the buffer circuit.

Further, preferably, a direction of length of a gate of a transistor asthe switch is made identical within the plurality of switch blocks.

A semiconductor integrated circuit according to a second embodiment ofthe present invention includes: a circuit block having an internalvoltage line; a first annular rail line forming a closed annular linearound the circuit block and supplied with one of a power supply voltageand a reference voltage; a second annular rail line forming a closedannular line around the circuit block and connected to the internalvoltage line at a plurality of positions between the first annular railline and the circuit block; and a plurality of switch blocks arrangedaround the circuit block along the first annular rail line and thesecond annular rail line, the plurality of switch blocks each includinga first voltage line segment forming a part of the first annular railline, a second voltage line segment forming a part of the second annularrail line, and a switch connected between the first voltage line segmentand the second voltage line segment.

In the present invention, preferably, mutual positional relation betweenthe first voltage line segment, the second voltage line segment, and theswitch is uniform in the plurality of switch blocks.

In the present invention, preferably, positional relation of end sidesof the first voltage line segment and the second voltage line segment attwo opposed sides of a block frame is set identical in the plurality ofswitch blocks.

According to the constitution of the first embodiment described above, aswitch block is formed including a voltage line segment forming a partof an annular rail line and a switch, and a plurality of such switchblocks are arranged around a circuit block. Therefore, mutual positionalrelation between the annular rail line and the switch is uniform in theplurality of switch blocks.

Two voltage line segments are connected to each other by making switchblocks adjoin each other, or when the switch blocks are at a distancefrom each other, wiring is connected between the two voltage linesegments, whereby a closed annular rail line is formed around thecircuit block.

According to the constitution of the second embodiment described above,two annular rail lines referred to as a first annular rail line and asecond annular rail line are formed.

Thus, each of a plurality of switch blocks includes a first voltage linesegment forming a part of the first annular rail line and a secondvoltage line segment forming a part of the second annular rail line. Aswitch is connected between the first voltage line segment and thesecond voltage line segment. In this case, preferably, mutual positionalrelation between the first annular rail line, the second annular railline, and the switch is uniform in the plurality of switch blocks.

Therefore, first voltage line segments are connected to each other andsecond voltage line segments are connected to each other by makingswitch blocks adjoin each other, or when the switch blocks are at adistance from each other, connection is established between thecorresponding voltage line segments by wiring, whereby two closedannular rail lines are formed around a circuit block.

When a switch block is to be moved, constituent elements other than thevoltage line segment of the switch block to be moved are moved to adesired position, and the constituent elements are connected to theannular rail line at the position to which the constituent elements havebeen moved.

At this time, in the first embodiment, a change in connection to thecircuit block can be necessary. However, when each switch block has apart of the first annular rail line and a part of the second annularrail line as in the second embodiment, in particular, a change inconnection to the circuit block is not necessary. Thus, by merely movingconstituent elements other than the first voltage line segment and thesecond voltage line segment of a switch block on each of the firstannular rail line and the second annular rail line, the switch block canbe moved freely on the power supply line rails.

The above is a description of effects in moving a switch. Also in a caseof switch insertion or deletion, it similarly suffices to insert ordelete a necessary number of constituent elements other than the voltageline segments (or the first voltage line segments and the second voltageline segments) of switch blocks at necessary positions.

According to the embodiment of the present invention, in moving a switchor changing the number of switches, it suffices principally to changeconstituent elements other than the voltage line segment of the switchblock. Therefore benefits are obtained in that it is easy to make adesign change and it is easy to optimize the position and the number ofswitches. More specifically, in a case where a power supply voltagesupply line or a reference voltage supply line is separated from theswitches and there is no uniformity in mutual positional relationbetween the supply line and the switches, metal wiring layers atdifferent levels need to be connected to each other again in making adesign change such as the moving of a switch, the insertion or deletionof a switch or the like. On the other hand, the present inventioneliminates a need for connecting every piece of such wiring other thanthe annular rail line again.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a general configuration of asemiconductor integrated circuit according to an embodiment of thepresent invention;

FIG. 2 is a diagram of internal configuration of a power shutoff objectcircuit block according to the embodiment;

FIG. 3 is a diagram showing a first example of rail arrangement of theembodiment;

FIG. 4 is a diagram showing a second example of rail arrangement of theembodiment;

FIG. 5 is a diagram showing a circuit block and a control line routed toswitch blocks around the circuit block in the embodiment;

FIG. 6 is a block diagram showing a configuration without an annularrail line;

FIG. 7 is a diagram schematically showing an arrangement of switchtransistors in the embodiment;

FIGS. 8A and 8B relate to the embodiment, FIG. 8A being a diagram of aconfiguration of an upper switch block, and FIG. 8B being a diagram of aconfiguration of a lower switch block;

FIGS. 9A and 9B relate to the embodiment, FIG. 9A being a diagram of aconfiguration of a left switch block, and FIG. 9B being a diagram of aconfiguration of a right switch block;

FIG. 10 is a diagram showing use conditions of wiring layers atrespective levels in a multilayer wiring structure by references 1M to7M;

FIG. 11 is a schematic layout diagram of a switch cell according to theembodiment;

FIG. 12 is a schematic layout diagram of another switch cell accordingto the embodiment;

FIGS. 13A and 13B relate to the embodiment, FIG. 13A being a diagram ofa configuration of an upper switch block formed using the switch cell ofFIG. 11, and FIG. 13B being a diagram of a configuration of a lowerswitch block;

FIGS. 14A and 14B relate to the embodiment, FIG. 14A being a diagram ofa configuration of a left switch block formed using the switch cell ofFIG. 11, and FIG. 14B being a diagram of a configuration of a rightswitch block; and

FIG. 15 is a diagram showing an appropriate arrangement of switch blocksin the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will hereinafter bedescribed with reference to the drawings.

<General Configuration>

FIG. 1 shows a general configuration of a semiconductor integratedcircuit according to an embodiment of the present invention.

In FIG. 1, a plurality of input-output cells 40 are respectivelyarranged so as to form a column along four sides of a rectangularsemiconductor chip on which the semiconductor integrated circuit isformed.

A few circuit blocks are arranged in a chip area for circuit arrangementwhich area is surrounded on all four sides by the input-output cells 40shown in FIG. 1. In the example shown in FIG. 1, a basic configurationof the semiconductor integrated circuit which configuration is referredto as an “energized region,” for example an energized circuit block 32including a CPU, a register, a memory, a power supply circuit and thelike is disposed in the chip area. The energized circuit block 32corresponds to a circuit block to which the MTCMOS technology is notapplied. The energized circuit block 32 operates by being supplied witha power supply voltage VDD and a reference voltage VSS at all timesafter the semiconductor integrated circuit is started.

A large number of circuit blocks referred to as “macros” and including,in a part thereof or the whole thereof, a general-purpose circuitindividually designed so as to be also usable in other semiconductorintegrated circuits are further arranged in the chip area for circuitarrangement. The “macros” can be designed by outsourcing, and can bepurchased from other companies as IP (Intellectual Property).

The circuit blocks as “macros” can be roughly classified intonon-applied circuit blocks 33 referred to as “energized macros” to whichthe MTCMOS technology is not applied and which operate by being suppliedwith the power supply voltage VDD and the reference voltage VSS at alltimes after the semiconductor integrated circuit is started as with theenergized circuit block 32 and power shutoff object circuit blocks 1referred to as “power shutoff macros” to which the MTCMOS technology isapplied and power to which is shut off as demanded.

Incidentally, the energized circuit block 32, the non-applied circuitblocks 33, and the power shutoff object circuit blocks 1 arranged in thechip area surrounded by the input-output cells 40 have a pair of a realVDD line and a real VSS line arranged therein, which are not shown inFIG. 1, and are thereby supplied with power. More specifically, some ofthe input-output cells 40 are assigned for power supply, and a pair ofreal power supply lines is arranged from the input-output cells 40 forpower supply in each of a row direction and a column direction withinthe chip area. Thereby power supply wiring of the energized circuitblock 32, the non-applied circuit blocks 33, and the power shutoffobject circuit blocks 1 is formed.

The power shutoff object circuit blocks 1 are of the so-called “externalSW arrangement type,” with switches controlling power shutoff andconnection being arranged around the power shutoff object circuit blocks1. As shown in FIG. 1, a predetermined number of switch blocks 2including the switches are arranged around the power shutoff objectcircuit blocks 1.

Though not shown in FIG. 1, an annular rail line to which the powersupply voltage VDD or the reference voltage VSS is applied is arrangedin such a manner as to be superimposed on the plurality of switch blocks2 arranged around the power shutoff object circuit blocks 1. At leastone annular rail line or preferably two annular rail lines are provided.An arrangement relation between the annular rail line and the switchblocks 2 will be described in the following with reference to drawings.

As described above, in the MTCMOS technology, a switch transistor isprovided at three kinds of positions, that is, a position between afunctional circuit that is started and stopped repeatedly and a real VDDline, a position between the functional circuit and a real VSS line, andboth the positions. In general, a PMOS transistor is used as a switch onthe VDD side, and an NMOS transistor is used as a switch on the VSS lineside.

A switch transistor in the present embodiment may be provided at any ofthe three kinds of positions described above. However, providing aswitch transistor on both of the VDD side and the VSS side leads to agreat disadvantage of an increase in area occupied by the switchtransistors considering effect of providing the switch transistors onboth of the VDD side and the VSS side. Therefore a switch transistor isdesirably provided on one of the VDD side and the VSS side. In addition,the NMOS transistor has a higher driving power per unit gate width thanthe PMOS transistor. Therefore, more desirably, a switch transistor isprovided on the VSS side.

Hence, the following description supposes the provision of a switch(transistor) on the VSS side.

FIG. 2 shows an example of internal configuration of a power shutoffobject circuit block 1.

In the illustrated example of configuration, the inside of the powershutoff object circuit block 1 is divided into a standard cellarrangement region 1A in which a functional circuit is realized by astandard cell and a macro cell region 1B of a RAM, for example.Incidentally, for application of the present invention, a “circuitblock” as an object for power shutoff control does not need to have amacro, and may be formed by only a standard cell arrangement region 1A.

Internal voltage lines 11 that are referred to as so-called “virtual VSSlines” and to which the reference voltage VSS is applied are arranged inparallel with each other in each of a row direction and a columndirection on the standard cell arrangement region 1A and the macro cellregion 1B. The internal voltage lines 11 in the row direction and theinternal voltage lines 11 in the column direction are formed by a wiringlayer at a level higher than that of the cells, and are interconnectedat intersections thereof.

On the other hand, though not shown to avoid complexity, power supplylines that are referred to as a so-called “real VDD line” and to whichthe power supply voltage VDD is applied and power supply lines to whichthe reference voltage VSS is applied are similarly arranged in the formof a lattice.

In the standard cell arrangement region 1A, a plurality of branch lines11A extend in the row direction at predetermined intervals from columndirection trunk wiring of the internal voltage lines 11 as “virtual VSSlines.” In addition, in the standard cell arrangement region 1A, aplurality of branch lines 12A extend in the row direction atpredetermined intervals from column direction trunk wiring of thevoltage supply lines as “real VDD lines” not shown in FIG. 2.

FIG. 2 shows one inverter cell 13 in an enlarged state, the invertercell 13 representing standard cells. The inverter cell 13 has a VSS linesegment forming a part of a branch line 11A and a VDD line segmentforming a part of a branch line 12A. A PMOS transistor and an NMOStransistor are connected in series with each other between the twosegments. The gates of the PMOS transistor and the NMOS transistor areboth connected to an input signal line. A node between the PMOStransistor and the NMOS transistor is connected to an output signalline. The input signal line and the output signal line are formed bysignal line segments within the inverter cell 13 and adjacent cells.However, an input and an output line of the whole of the standard cellarrangement region 1A are formed by a wiring layer (not shown) at ahigher level.

<First Example of Rail Arrangement>

FIG. 3 shows a first example of rail arrangement.

As illustrated in FIG. 3, a plurality of switch blocks 2 are arranged soas to surround a power shutoff object circuit block 1 in proximity tothe four sides of the power shutoff object circuit block 1. In thiscase, for convenience, the switch blocks 2 are classified into upperswitch blocks 2U, lower switch blocks 2D, right switch blocks 2R, andleft switch blocks 2L for the respective sides of the power shutoffobject circuit block 1. The upper switch blocks 2U have a sameconfiguration; the lower switch blocks 2D have a same configuration; theright switch blocks 2R have a same configuration; and the left switchblocks 2L have a same configuration.

The four kinds of switch blocks 2U, 2D, 2R, and 2L each have a voltageline segment 21 and a switch not shown in FIG. 3.

The voltage line segment 21 is represented by a broken line in FIG. 3.The voltage line segment 21 is a wiring part forming a part of anannular rail line 3 that forms a closed annular line around the powershutoff object circuit block 1.

In a state of design of arrangement wiring, the annular rail line 3 isdisposed and connected to switches and the like at a wiring stage afterthe switches are arranged. After the annular rail line 3 is oncedisposed and connected, constituent elements (including a switch) otherthan a voltage line segment 21 are moved in a unit of a switch block 2U,2D, 2R, or 2L, and the switch and the like are connected to the annularrail line 3 at a position after the moving. Incidentally, in FIG. 3,connection wiring for connection between an internal voltage line 11(see FIG. 2) within the power shutoff object circuit block 1 and theswitch needs to be changed each time the switch block is moved. However,the trouble of changing connection wiring for connecting the annularrail line 3 to the switch is saved, and it is correspondingly easy tomove the switch.

Similarly, in a case of inserting switch blocks, a necessary number ofswitch blocks are inserted at necessary positions with constituentelements other than voltage line segments 21 within the switch blocks asunits, and switches and the like are connected to the annular rail line3 at the positions.

Similarly, in a case of deleting a switch, the switch is deleted withconstituent elements other than a voltage line segment 21 within theswitch block as a unit.

Even in inserting or deleting a switch, the trouble of changingconnection wiring for connecting the annular rail line 3 to the switchis saved, and it is correspondingly easy to move the switch.

<Second Example of Rail Arrangement>

FIG. 4 shows a second example of rail arrangement.

As a first difference of the example of arrangement shown in FIG. 4 fromFIG. 3, a virtual annular rail line 3V as a “second annular rail line”is provided in addition to an annular rail line 3 as a “first annularrail line.”

The virtual annular rail line 3V is disposed in parallel with theannular rail line 3 between the annular rail line 3 and the powershutoff object circuit block 1. The virtual annular rail line 3V isconnected to predetermined positions of internal voltage lines 11 (seeFIG. 2) within the power shutoff object circuit block 1, for exampleeach end part (three positions in the row direction and four positionsin the column direction) of the internal voltage lines 11 in the case ofFIG. 2.

In each of switch blocks 2U, 2D, 2R, and 2L, a switch not shown in thefigure is connected between the annular rail line 3 (voltage linesegment 21) and the virtual annular rail line 3V (virtual voltage linesegment 21V).

As a second difference, in each of the switch blocks 2U, 2D, 2R, and 2L,a virtual voltage line segment 21V as a “second voltage line segment” isprovided in parallel with a voltage line segment 21 as a “first voltageline segment.”

The other arrangement itself of the switch blocks 2U, 2D, 2R, and 2Lwith respect to the power shutoff object circuit block 1 is the same asin FIG. 3.

In the second example of rail arrangement, as in the first example ofrail arrangement, constituent elements other than the voltage linesegment 21 and the virtual voltage line segment 21V of a switch blockare only moved, inserted, or deleted. The trouble of changing connectionwiring for connecting the annular rail line 3 to the switch is saved,and it is correspondingly easy to move the switch.

Further, in the second example of rail arrangement, the virtual annularrail line 3V is connected to each end part (three positions in the rowdirection and four positions in the column direction) of the internalvoltage lines 11, for example. The trouble of changing connection wiringfor connecting the virtual annular rail line 3V to the switch is alsosaved with no change made to the connection positions, and it iscorrespondingly easy to move the switch.

As already described, in the first example of rail arrangement (FIG. 3)and the second example of rail arrangement (FIG. 4) described above, theposition and the number of switch blocks can be changed freely by onlymoving, inserting, or deleting constituent elements excluding thevoltage line segment 21 (and the virtual voltage line segment 21V) ofthe above-described switch block.

In order to enable this free design change, it is necessary that theswitch blocks 2U, 2D, 2R, and 2L have a same size and that end sidepositions of voltage line segments 21 (and virtual voltage line segments21V) at two opposed sides of block frames of the switch blocks 2U, 2D,2R, and 2L which opposed sides are crossed by the annular rail line 3(and the virtual annular rail line 3V) be standardized (fixed).

Incidentally, when the end side positions are not standardized, thepattern of the annular rail line 3 (and the virtual annular rail line3V) needs to be corrected such that end side positions are connected toeach other between adjacent switch blocks after constituent elementsexcluding the voltage line segment 21 (and the virtual voltage linesegment 21V) of a switch block are moved along the annular rail line 3(and the virtual annular rail line 3V), inserted, or deleted. However,this work is a simple operation of connecting end sides and can thus beautomated. Thus, switch arrangement is changed much more easily than ina case of manually performing again connection wiring connectingswitches to an annular line disposed outside the switches using a wiringlayer at another level after arranging the switches.

<Switch Control Line>

Though not shown in FIG. 3 or FIG. 4, a plurality of switch controllines may be arranged, depending on the number of switch groupscontrolled simultaneously.

FIG. 5 is a diagram showing an example of arrangement of two controllines. In this case, the second example of rail arrangement (FIG. 4) isadopted for annular rail lines.

A control circuit 34 shown in FIG. 5 is for example provided in anMTCMOS non-applied circuit block such as the energized circuit block 32or the non-applied circuit blocks 33 in FIG. 1. The control circuit 34can thereby operate at all times while supplied with power after thesemiconductor integrated circuit is started. A control line 35 from thecontrol circuit 34 is routed to upper switch blocks 2U, left switchblocks 2L, lower switch blocks 2D, and right switch blocks 2R in thisorder, and a control signal is applied in this order. The control signalcontrols the conduction and non-conduction of a switch within eachswitch block.

Incidentally, the configuration shown in FIG. 5 is provided withbranches of real VSS wiring at predetermined positions of an annularrail line 3 so as to be usable as a replacement for a configurationwithout an annular rail line which configuration is shown in FIG. 6.

In the external SW arrangement configuration shown in FIG. 6 to whichthe present invention is not applied, pieces of global real VSS wiring 5provided in the form of a lattice are connected to a power shutoffobject circuit block 1 via switch blocks SB.

The annular rail line 3 shown in FIG. 5 is connected to real VSS wiring5 arranged in the form of a lattice at two positions in the rowdirection and four positions in the column direction.

On the other hand, a virtual annular rail line 3V is connected to apower shutoff object circuit block 1 at six positions in the rowdirection and eight positions in the column direction.

These connection positions do not need to be changed at all when aswitch block 2U, 2D, 2R, or 2L is moved, inserted, or deleted.

A more detailed switch configuration in an example in which switchcontrol is performed by two control lines 35 will next be described withreference to a drawing.

<Switch Configuration Example>

FIG. 7 is a switch block configuration diagram schematically showing anarrangement of switch transistors between a voltage line segment 21 towhich the reference voltage VSS (for example 0 [V]) is applied and avirtual voltage line segment 21V maintained at a virtual referencevoltage VSSV.

In FIG. 7, three branch lines 21B are provided to the voltage linesegment 21, and three branch lines 21VB are provided to the virtualvoltage line segment 21V. The branch lines 21B and the branch lines 21VBare arranged alternately with each other. Four switch transistors SWTare connected in parallel with each other between one branch line 21Band one branch line 21VB adjacent to the branch line 21B. Five stages ofsuch switch transistor strings are provided, and a total of 4×5=20switch transistors SWT are arranged in the form of a matrix.

One control line not shown in FIG. 7 controls the conduction andnon-conduction of one switch transistor SWT in each stage or a total offive switch transistors SWT of the 20 switch transistors SWT. Thecircuit symbol parts of the five switch transistors SWT are shown with adark mesh in FIG. 7. The other 15 switch transistors SWT not providedwith the mesh are simultaneously controlled by another control line.

A part of the switches and the other switches are thus controlledseparately from each other in order to suppress variations in potentialof the voltage line segment 21 (annular rail line 3) due to suddenswitching when power supply is started to return from a stopped state inwhich power supply to the power shutoff object circuit block 1 (seeFIGS. 1 to 5) is shut off to an operating state. Thus, control isperformed such that a small number of switch transistors SWT or the fiveswitch transistors SWT in this case are turned on first to lower thepotential of the virtual voltage line segment 21V (internal voltagelines 11 within the power shutoff object circuit block 1) to a certaindegree by a relatively high on resistance, and when the potential of thevirtual voltage line segment 21V is stabilized, the other 15 switchtransistors SWT are turned on. Thereby, the peak value of a potentialrise (power supply noise) of the reference voltage VSS transmitted fromthe annular rail line 3 to the real VSS wiring 5 is suppressed to such alevel as not to affect other circuits operating at all times.

FIGS. 8A to 9B show an example of an arrangement within an actual switchblock. In terms of correspondence with FIG. 3 and FIG. 4, FIG. 8A showsan upper switch block 2U, FIG. 8B shows a lower switch block 2D, FIG. 9Ashows a left switch block 2L, and FIG. 9B shows a right switch block 2R.

The four kinds of switch blocks 2U, 2D, 2R, and 2L have a same size. Inthis case, the sides (hereinafter referred to as connection sides) of ablock frame at which sides the reference voltage VSS, the virtualreference voltage VSSV, and the control signal are input and output,that is, sides LU1 and LU2 in FIG. 8A, sides LD1 and LD2 in FIG. 8B,sides LL1 and LL2 in FIG. 9A, and sides LR1 and LR2 in FIG. 9B are setat a same length. In addition, the respective end sides of a voltageline segment 21, a virtual voltage line segment 21V, a first switchcontrol line 35_1, and a second switch control line 35_2 arestandardized at same positions at any of these connection sides.

In this case, the first switch control line 35_1 controls a number ofswitch transistors SWT controlled first as in FIG. 7, and the secondswitch control line 35_2 controls a number of other switch transistorsSWT.

Unlike FIG. 7, the switch blocks 2U, 2D, 2R, and 2L shown in FIGS. 8Aand 8B and FIGS. 9A and 9B have the voltage line segment 21 and thevirtual voltage line segment 21V arranged in parallel with each other insuch a manner as to be superimposed above a switch arrangement regionenclosed by a broken line. This provides an advantage of reducing anarea occupied by the blocks. However, an arrangement in which thevoltage line segment 21 and the virtual voltage line segment 21V are notsuperimposed above a switch arrangement region as in FIG. 7 can also beadopted.

When the voltage line segment 21 and the virtual voltage line segment21V are arranged in parallel with each other in such a manner as to besuperimposed above the switch arrangement region, the first switchcontrol line 35_1 and the second switch control line 35_2 may not bearranged in the switch arrangement region with a limited number oflayers of multilayer wiring structure. Hence, in the present example,the first switch control line 35_1 and the second switch control line35_2 are disposed outside the switch arrangement region on an outer sideopposite from the power shutoff object circuit block 1.

The left switch block 2L and the right switch block 2R shown in FIGS. 9Aand 9B have six switch cells as shown in FIG. 7 in an X-direction(horizontal direction of the figures) and six switch cells as shown inFIG. 7 in a Y-direction (vertical direction of the figures), that is, atotal of 36 switch cells arranged therein.

On the other hand, while the upper switch block 2U and the lower switchblock 2D shown in FIGS. 8A and 8B have a total of 36 switch cells, whichnumber is the same as in FIGS. 9A and 9B, the upper switch block 2U andthe lower switch block 2D have 12 switch cells arranged in theX-direction and three switch cells arranged in the Y-direction.

The reason is that a switch cell has a large size in the Y-direction ascompared with that in the X-direction, and that to meet a need to makethe direction of length of gate electrodes the same Y-direction in FIGS.8A and 8B and FIGS. 9A and 9B, horizontal-to-vertical ratios of switcharrangement regions contained within switch blocks having the same sizeare adjusted to the vertical size and the horizontal size of the switchblocks, the vertical size and the horizontal size of the switch blocksbeing interchanged in FIGS. 8A and 8B and FIGS. 9A and 9B.

In the case of FIGS. 9A and 9B, the voltage line segment 21 and thevirtual voltage line segment 21V each intersect all branch lines 21B and21VB. Thus, the voltage line segment 21 can be connected to all thebranch lines 21B and 21VB in a lower layer via contacts, and the virtualvoltage line segment 21V can be connected to all the branch lines 21Band 21VB in the lower layer via contacts.

In the case of FIGS. 8A and 8B, on the other hand, the voltage linesegment 21 and the virtual voltage line segment 21V do not intersect allthe lower layer wiring (branch lines 21B and 21VB) to which to providecontacts. Thus, as shown in FIGS. 8A and 8B, in the upper switch block2U and the lower switch block 2D, access branch lines 21Ba orthogonal tothe voltage line segment 21 and the branch lines 21B need to be providedin an access path from the voltage line segment 21 to the branch lines21B. The access branch lines 21Ba are formed by a wiring layer lowerthan the voltage line segment 21 and higher than the branch lines 21B.Thus, a two-stage branch structure is adopted in which the branch linesof the voltage line segment 21 are the access branch lines 21Ba andfurther the branch lines 21B branch off from the access branch lines21Ba.

Similarly, a two-stage branch structure is adopted in which accessbranch lines 21VBa orthogonal to the virtual voltage line segment 21Vbranch off from the virtual voltage line segment 21V and further thebranch lines 21VB orthogonal to the access branch lines 21VBa branch offfrom the access branch lines 21VBa.

The total gate width (total length in the direction of length) of theswitch transistors SWT controlled by the first switch control line 35_1is set to be the same in FIGS. 8A and 8B and FIGS. 9A and 9B. Similarly,the total gate width of the switch transistors SWT controlled by thesecond switch control line 35_2 is set to be the same in FIGS. 8A and 8Band FIGS. 9A and 9B.

Each of the switch blocks 2U, 2D, 2R, and 2L has a buffer circuit BUF1provided at an intermediate point of the first switch control line 35_1and a buffer circuit BUF2 provided at an intermediate point of thesecond switch control line 35_2 on an opposite side (outside) of theswitch arrangement region from the circuit block 1.

The buffer circuits BUF1 and BUF2 are connected to the real VDD line notshown in the figure and the voltage line segment 21 to thereby perform afunction of shaping the waveform of the control signal attenuated in aprocess of being transmitted into that of a pulse signal having theamplitude of the power supply voltage VDD. Thus, a buffer circuitarrangement region represented by a broken line is provided on theoutside of the switch arrangement region.

Wiring for switch control extends from each output of the buffercircuits BUF1 and BUF2 to the switch arrangement region, and isconnected to the gate of a corresponding switch transistor group.

Incidentally, this wiring as well as the first switch control line 35_1and the second switch control line 35_2 is represented by a line inFIGS. 8A and 8B and FIGS. 9A and 9B, but is actually formed by a wiringlayer having a similar width to that of the voltage line segment 21 andthe like.

The switch control line 35_1 and the second switch control line 35_2each include, within the switch block, a “first control line segment”that is provided on the input side of the corresponding buffer circuitBUF1 or BUF2 and to which the control signal is input and a “secondcontrol line segment” that is provided on the output side of thecorresponding buffer circuit BUF1 or BUF2 and to which thewaveform-shaped control signal is output.

<Wiring Structure>

FIG. 10 shows an example of use of wiring layers at respective levels ina multilayer wiring structure. In this case, each piece of wiring isformed using a wiring layer at a first level (a first wiring layer (1M))to a wiring layer at a seventh level (a seventh wiring layer (7M))laminated in order from the bottom layer in the multilayer wiringstructure.

Specifically, wiring between standard cells 15 such as the inverter cell13 (FIG. 2) and the like within the power shutoff object circuit block 1is formed of the first wiring layer (1M) to the fourth wiring layer(4M). A signal line routed from a certain standard cell 15 to theoutside is also formed of the first wiring layer (1M) to the fourthwiring layer (4M).

Wiring 3C in the column direction of the annular rail line 3 is formedof the fifth wiring layer (5M). Wiring 3R in the row direction of theannular rail line 3 is formed of the sixth wiring layer (6M) higher byone level than the fifth wiring layer (5M) and connected to both ends ofthe wiring 3C in the column direction.

Similarly, wiring 3VC in the column direction of the virtual annularrail line 3V is formed of the fifth wiring layer (5M). Wiring 3VR in therow direction of the virtual annular rail line 3V is formed of the sixthwiring layer (6M) higher by one level than the fifth wiring layer (5M)and connected to both ends of the wiring 3VC in the column direction.

Wiring 11R in the row direction of the internal voltage lines 11 isformed of the sixth wiring layer (6M) higher by one level than the fifthwiring layer (5M) to be connected to the wiring 3VC in the columndirection of the virtual annular rail line 3V, the wiring 3VC beingformed of the fifth wiring layer (5M). Further, the wiring 11R in therow direction of the internal voltage lines 11, the wiring 11R beingformed of the sixth wiring layer (6M), is connected to wiring 11C in thecolumn direction of the internal voltage lines 11, the wiring 11C beingformed of the seventh wiring layer (7M) higher by one level than thesixth wiring layer (6M), at intersections of the wiring 11R and thewiring 11C.

Incidentally, the real VSS wiring 5 is also formed by the seventh wiringlayer (7M).

Thus, inter-wiring connection is achieved well by forming wiring in thecolumn direction out of wiring lower by one level than wiring in the rowdirection and applying this rule.

In the above description, the pattern of a switch cell is arbitrary andhas not been mentioned. However, a switch cell having a biaxialsymmetric pattern will next be described as a desirable concreteexample.

FIG. 11 is a schematic layout diagram of one switch cell. Incidentally,pattern size and the like in FIG. 11 are not equal to those of an actualswitch cell, and FIG. 11 only schematically shows the rough arrangementand connecting relation of the pattern.

The switch cell 20N shown in FIG. 11 is formed by converting one NMOStransistor into a standard cell. The switch cell 20N has a configurationsymmetric with respect to each of an X-axis and a Y-axis passing throughthe center of the cell. This symmetry will be referred to as “biaxialsymmetry.”

The entire region of the switch cell 20N illustrated in FIG. 11 forms apart of a P-well.

A gate electrode coupling part 21C passing through the center of thecell along the X-axis is formed. The lengths of the coupling part 21Cfrom the center of the cell are the same (symmetric) with the Y-axis asa boundary. That is, the coupling part 21C has a pattern of biaxialsymmetry.

Four gate electrodes 21A of a same length extend from the coupling part21C on one side in a width direction of the coupling part 21C, and fourgate electrodes 21B of the same length extend from the coupling part 21Con another side of the coupling part 21C. The four gate electrodes 21Aare arranged at equal intervals in the direction of the X-axis. The fourgate electrodes 21B are similarly arranged at equal intervals in thedirection of the X-axis. Because the gate electrodes 21A and the gateelectrodes 21B all have the same length and a same thickness, the gateelectrodes 21A and the gate electrodes 21B are symmetric with respect tothe Y-axis. Because the gate electrodes 21A and 21B branch off from thesame positions of the coupling part 21C, the gate electrodes 21A and 21Bare symmetric with respect to the X-axis. The coupling part 21C and thegate electrodes 21A and 21B are formed integrally with each other bytreating a same conductive material.

Two N-type active regions 22A and 22B of a same size are formed in theP-well at equal distances from the X-axis. The N-type active region 22Ais formed in a position in which the N-type active region 22A intersectsthe four gate electrodes 21A. The N-type active region 22B is formed ina position in which the N-type active region 22B intersects the fourgate electrodes 21B. The N-type active regions 22A and 22B are formed byselectively introducing an N-type impurity into the P-well with the gateelectrodes 21A and 21B as a mask after the gate electrodes 21A and 21Bare formed.

The N-type active regions 22A and 22B are each divided by the parts ofthe gate electrodes into five regions alternately functioning as asource (S) and a drain (D).

Thereby, a basic structure having biaxial symmetry is formed whichstructure includes a first unit transistor (TR1) having the partsdividing the N-type active region 22A as a channel and a second unittransistor (TR2) having the parts dividing the N-type active region 22Bas a channel.

In the region in which the first unit transistor (TR1) is disposed, avoltage cell line 23A formed of the second wiring layer (2M) is disposedso as to be orthogonal to the four gate electrodes 21A. Similarly, inthe region in which the second unit transistor (TR2) is disposed, avoltage cell line 23B formed of the second wiring layer (2M) is disposedso as to be orthogonal to the four gate electrodes 21B.

The two voltage cell lines 23A and 23B are each a cell internal lineelectrically connected to an internal voltage line 11 within the circuitblock 1 (see FIG. 2) by the virtual voltage line segment 21V (see FIG.7) in a layer higher than the voltage cell lines 23A and 23B. That is,in terms of correspondence with FIG. 7, the two voltage cell lines 23Aand 23B in FIG. 11 correspond to one branch line 21VB in FIG. 7.

Two drain lines 28 connected to two respective drains (D) via a 1stcontact (1C) in each of the N-type active regions 22A and 22B areprovided. The two drain lines 28 in each of the N-type active regions22A and 22B or a total of four drain lines 28 are formed by the firstwiring layer (1M).

The voltage cell line 23A is connected to the two drain lines 28 on theN-type active region 22A via a 2nd contact (2C). Similarly, the voltagecell line 23B is connected to the two drain lines 28 on the N-typeactive region 22B via a 2nd contact (2C).

The two voltage cell lines 23A and 23B are parallel with each other, andarranged at equal distances from the X-axis.

A power cell line 24A parallel with the voltage cell line 23A isdisposed on a tip side of the four gate electrodes 21A. Similarly, apower cell line 24B parallel with the voltage cell line 23B is disposedon a tip side of the four gate electrodes 21B.

The power cell lines 24A and 24B are each a cell internal lineelectrically connected to the real VDD line (the voltage line segment 21in FIG. 7) in a higher layer than the power cell lines 24A and 24B. Thatis, in terms of correspondence with FIG. 7, the two power cell lines 24Aand 24B in FIG. 11 correspond to one branch line 21B in FIG. 7.

The two power cell lines 24A and 24B each include a wiring region 24 dformed simultaneously with a P-type active region not shown in thefigure and the like, first lining wiring 24 m 1 formed of the firstwiring layer (1M), and second lining wiring 24 m 2 formed of the secondwiring layer (2M).

In each of the two power cell lines 24A and 24B, the wiring region 24 dand the first lining wiring 24 m 1 are short-circuited by a 1st contact(1C) at equal intervals, and the first lining wiring 24 m 1 and thesecond lining wiring 24 m 2 are short-circuited by a 2nd contact (2C) atequal intervals.

The first lining wiring 24 m 1 forming the power cell line 24A is formedintegrally with two source lines 24S extending on two source (S) sidesof the N-type active region 22A. Similarly, the first lining wiring 24 m1 forming the power cell line 24B is formed integrally with two sourcelines 24S extending on two source (S) sides of the N-type active region22B.

The sources (S) are connected to the source lines 24S via a 1st contact(1C).

In this case, the gate electrode coupling part 21C described first canbe omitted, and is replaceable by four contact pad parts.

In either case, as a whole, the four gate electrodes formed by the gateelectrodes 21A and 21B parallel with the Y-axis are short-circuited by awiring layer higher than the gate electrodes. Cell internal wiringshort-circuiting the gate electrodes will be referred to as a “controlcell line.”

The control cell line 25 in the present example is formed bysuperimposing a second control cell line 27 formed of the second wiringlayer (2M) on a first control cell line 26 formed of the first wiringlayer (1M). The coupling part 21C (or the four contact pad parts) andthe first control cell line 26 are connected to each other by a 1stcontact (1C). The first control cell line 26 and the second control cellline 27 are connected to each other by a 2nd contact (2C).

The control cell line 25 is disposed along the X-axis with the centersin the direction of width and the direction of length of the controlcell line 25 coinciding with the center of the cell.

Thus, the control cell line 25 is disposed in parallel with each of thetwo voltage cell lines 23A and 23B and the two power cell lines 24A and24B.

FIG. 12 shows a switch cell 20P of a PMOS transistor.

The switch cell 20P illustrated in FIG. 12 differs from the switch cell20N of FIG. 11 in that the whole of the switch cell 20P is formed in anN-well and the active regions of a first unit transistor (TR1) and asecond unit transistor (TR2) formed in the N-well are P-type activeregions 22AP and 22BP. Other configuration is the same as in FIG. 11.Therefore, the same reference numerals are provided to the otherconfiguration, and description thereof will be omitted.

FIGS. 13A and 13B are diagrams of configuration of an upper switch block2U and a lower switch block 2D using the switch cell 20N of the NMOStransistor configuration in FIG. 11. FIGS. 14A and 14B are diagrams ofconfiguration of a left switch block 2L and a right switch block 2Rsimilarly using the switch cell 20N.

A comparison of FIGS. 13A and 13B and FIGS. 14A and 14B with FIGS. 8Aand 8B and FIGS. 9A and 9B shows that power cell lines 24ABcorresponding to the branch lines 21B in FIGS. 8A and 8B and FIGS. 9Aand 9B are formed at twice the density of the branch lines 21B and thatvoltage cell lines 23A and 23B corresponding to the branch lines 21VB inFIGS. 8A and 8B and FIGS. 9A and 9B are formed at twice the density ofthe branch lines 21VB. In this case, a power cell line 24AB is shared bythe power cell line 24A in FIG. 11 and the power cell line 24B ofanother cell adjacent in the Y-direction.

Other configuration is common, and therefore description thereof will beomitted.

Though not shown in FIGS. 13A and 13B and FIGS. 14A and 14B, amultilayer wiring structure is used to connect a predetermined number ofswitch cells 20N at predetermined positions to each of the first switchcontrol line 35_1 and the second switch control line 35_2. When wiringlayers are increased in number, the switch control lines can be arrangedsymmetrically with respect to the center of the blocks. It is notdesirable, however, to complicate the wiring structure and thus increasemanufacturing cost only for this purpose. In addition, in the presentexample, from a need to dispose the buffer circuits BUF1 and BUF2, it isnecessary to arrange the first switch control line 35_1 and the secondswitch control line 35_2 asymmetrically with respect to the center ofthe switch blocks.

Description will next be made of advantages of the symmetric wiringstructure of switch cells when the switch control lines thus may not bearranged symmetrically with respect to the center of the blocks.

Description will first be made of the ease of design of switch blocks asa first advantage.

A preferable switch arrangement wiring method (switch block designingmethod) in the present embodiment follows the following procedureutilizing layout symmetry of the switch cell 20N.

First step: Transistors are arranged, and each of two voltage cell lines23A and 23B to be each electrically connected to internal voltage lines11, two power cell lines 24A and 24B (or two shared power cell lines24AB) to be each electrically connected to a second power supply line towhich the power supply voltage VDD (in the case of the switch cell 20P)or the reference voltage VSS (in the case of the switch cell 20N) isapplied, and a control cell line 25 to be electrically connected toswitch control lines 29A to 29C is arranged symmetrically with respectto each of the X-axis and the Y-axis passing through the center of thecell, and is connected to the transistors. Thereby the switch cell 20Nor 20P (or both thereof) is formed.

Second step: Formed switch cells 20N or 20P (or both thereof) arearranged in the form of a matrix. Predetermined switch cells 20N or 20P(or both thereof) are connected to each of the plurality of switchcontrol lines 29A to 29C. Thereby a switch block 20 is formed.

Third step: Created data on the switch block 2 is mirror-inverted on aline parallel to the X-axis or the Y-axis, or rotated by 180 degrees(inverted by 180 degrees) on a cell center. Thereby an inverted switchblock is formed.

Fourth step: The plurality of switch control lines 29A to 29C and thesecond power supply line are connected between the disposed switch blockand the inverted switch block. The voltage cell lines 23A and 23B areconnected to the internal voltage lines 11 of the circuit block 1.

Making description more specifically, though the description is arepetition, the control cell line 25, the voltage cell lines 23A and23B, and the power cell lines 24A and 24B (or the two power cell lines24AB) are symmetric with respect to each of the X-axis and the Y-axis.Thus, even when the switch cell 20N is mirror-inverted with a line alongthe X-axis or the Y-axis as an inversion axis or rotated by 180 degreeson a cell center, positional relation between the five cell linesdescribed above remains in the original state.

In the case of FIG. 5 in which even transistors are arranged biaxiallysymmetrically, in particular, even when the whole of a certain switchblock 2 is mirror-inverted or rotated by 180 degrees in a stage in whichdesign up to switch control lines is made as in FIG. 4, for example, thebasic pattern of each switch cell 20N as shown in FIG. 11 in a switchcell group in the form of a matrix is not changed at all. A change ismade to the switch control lines 29A to 29C and connecting linesthereof, which are not arranged symmetrically within the block and areformed by the third wiring layer (3M) and higher wiring layers.

Wiring between blocks is easy when the distance of each of the firstswitch control line 35_1 and the second switch control line 35_2 fromthe circuit block 1 is the same. In addition, there is often arestriction demanding that the orientation (direction of length) of thegates of transistors be the same within an integrated circuit foruniform characteristics. In such a case, switch blocks arranged on thefour sides of the circuit block 1 have a different pattern for eachside.

However, when switch cells as shown in FIG. 11 are used, in which switchcells the five cell lines, that is, the voltage cell lines 23A and 23B,the power cell lines 24A and 24B, and the control cell line 25 havebiaxial symmetry, switch blocks can be designed easily by the methodhaving the procedure of the first to fifth steps described above.

Utilizing the fact that the relation between the five cell lines is notchanged in consequence of mirror inversion or 180-degree rotation andwiring in higher layers than the cell lines is changed, after one switchblock 2 to be disposed on one side of two opposed sides of the circuitblock 1 is designed in the first and second steps, data on the switchblock 2 after the design is mirror-inverted on a line parallel to thetwo sides or rotated by 180 degrees. Thereby data on another switchblock 2 to be disposed on the other side can be created easily (thethird step).

Similarly, for the other two sides, after a switch block 2 to bedisposed on one of the sides is designed in the first and second steps,data after the design is mirror-inverted or rotated by 180 degrees (thethird step). Thereby data on a switch block 2 to be disposed on theother side can be created easily.

Each of the switch control lines 29A to 29C in the four kinds of switchblocks (2U, 2D, 2L, and 2R) thus created have the same distance to thecircuit block 1. It is therefore easy to connect the switch controllines between blocks in the fourth step. In addition, this is true forother wiring to be connected between switch blocks.

Description will next be made of the ease of switch cell design itself.

When even the pattern of transistors has biaxial symmetry as in FIG. 11and FIG. 12 in addition to the symmetry of the five cell lines, one ofpatterns of a first to a fourth quadrant divided by the X-axis and theY-axis (which patterns will hereinafter be referred to as quarterdivided patterns) is designed, and thereafter the other three quarterdivided patterns are formed by merely copying pattern data after thedesign and pasting the pattern data while performing mirror inversion ora combination of mirror inversion and 180-degree rotation. Thereby thedesign of a switch cell is completed.

Thus, a switch cell can be designed very easily. In addition, whenhigh-density design is made such that a maximum gate width can besecured in a stage of the first design of a quarter divided pattern, aswitch cell can be designed without a waste.

According to the present embodiment, the following benefits areobtained.

A plurality of switch blocks arranged around a power shutoff objectcircuit block 1 have a switch and a voltage line segment as a part of anannular rail line to which power supply voltage or reference voltage isapplied. Thus, a switch segment can be, as it were, freely moved alongthe annular rail line, newly inserted, or deleted easily by merelymoving, inserting, or deleting the switch block in a state in whichpositional relation between the voltage line segment and the switch isfixed.

In particular, as shown in FIG. 15, a plurality of switch blocks arearranged such that the lower the impedance of wiring (specifically thereal VSS wiring 5 and the annular rail line 3 in the description of thepresent embodiment) supplying the power supply voltage or the referencevoltage to the power shutoff object circuit block 1 as viewed from eachof four sides of the circuit block 1, the higher the number of switchblocks.

In FIG. 15, the impedance of a side indicated by a thick arrow is lowerthan that of a side indicated by a thin arrow. That is, becauseinput-output cells 40 shown in FIG. 1 are arranged on the side of thethick arrow, the external reference voltage assumes a value ofapproximately 0 [V]. On the other hand, because another circuit block(for example the energized circuit block 32) operating at all times isdisposed on the side indicated by the thin arrow, a time average of thepotential of a real VSS line is higher than 0 [V].

In such a case, when many switch blocks are disposed on the side wherethe reference voltage is fixed at 0 [V], the discharging of internalvoltage lines 11 progresses more efficiently within a same switch-ontime. On the other hand, when many switches are disposed on the sidewhere the reference voltage is higher than 0 [V], the number of switchblocks needs to be increased to obtain the same discharging effect, thusleading to wastefulness.

The present embodiment has an effect of enabling such an efficientswitch block arrangement to be made easily.

Specifically, provisions can be made at a time of determining powerconsumption in a latter half of design, and the number of switch blocksbeing used can be reduced as compared with the existing case. When thetotal gate width of switch transistors is reduced by decreasing thenumber of switch blocks, a leakage current is correspondingly decreased,so that a power reduction effect is obtained. In addition, because thereis no operating circuit block on the side of the input-output cells 40,there is a small effect of power supply noise due to a discharge, and anadverse effect on the operating speed of other circuit blocks can besuppressed.

In addition, because the annular line to which the power supply voltageor the reference voltage is applied can be disposed so as to besuperimposed on the switch, a great effect of area reduction isobtained.

Further, when switch cells having the biaxially symmetric pattern areused, the first advantage (the ease of switch block design) and thesecond advantage (the ease of switch cell design itself) described aboveare obtained.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A semiconductor integrated circuit comprising: a circuit block havingan internal voltage line; an annular rail line forming a closed annularline around said circuit block and supplied with one of a power supplyvoltage and a reference voltage; and a plurality of switch blocksarranged around said circuit block along said annular rail line, theplurality of switch blocks each including a voltage line segment forminga part of said annular rail line and a switch for controlling connectionand disconnection between said voltage line segment and said internalvoltage line, wherein said plurality of switch blocks are standardizedby having an identical size and including four different kinds of switchblocks for four respective sides of said circuit block, and wherein forsaid plurality of switch blocks, a number of switch blocks arranged oneach side of said circuit block is determined such that the number ofswitch blocks is increased as impedance of wiring supplying one of thepower supply voltage and the reference voltage to said circuit block asviewed from each of the four sides of said circuit block is decreased.2. The semiconductor integrated circuit according to claim 1, whereinsaid plurality of switch blocks have said voltage line segment and saidswitch internally connected to each other, and positional relation ofend sides of said voltage line segment at two opposed sides of a blockframe is set identical in said plurality of switch blocks.
 3. Thesemiconductor integrated circuit according to claim 1, wherein adirection of length of a gate of a transistor as said switch is madeidentical within said plurality of switch blocks.
 4. The semiconductorintegrated circuit according to claim 1, wherein in each of said switchblocks, said voltage line segment is disposed in one of states ofintersecting and being in proximity to an arrangement region of saidswitch, a buffer circuit operating to receive voltage supplied from saidvoltage line segment is formed on an opposite side of the arrangementregion of said switch and said voltage line segment from the circuitblock, a first control line segment to which a control signal is inputvia a control line is disposed on an input side of said buffer circuit,and a second control line segment electrically connected to a controlnode of said switch is disposed on an output side of said buffercircuit.
 5. The semiconductor integrated circuit according to claim 1,wherein a wiring part of said annular rail line which wiring part isparallel with one pair of opposed sides of four sides of said circuitblock and a wiring part of said annular rail line which wiring part isparallel with another pair of opposed sides of the four sides of saidcircuit block are formed by wiring layers at different levels.
 6. Asemiconductor integrated circuit comprising: a circuit block having aninternal voltage line; an annular rail line forming a closed annularline around said circuit block and supplied with one of a power supplyvoltage and a reference voltage; and a plurality of switch blocksarranged around said circuit block along said annular rail line, theplurality of switch blocks each including a voltage line segment forminga part of said annular rail line and a switch for controlling connectionand disconnection between said voltage line segment and said internalvoltage line, wherein in each of said switch blocks, said voltage linesegment is disposed in one of states of intersecting and being inproximity to an arrangement region of said switch, a buffer circuitoperating to receive voltage supplied from said voltage line segment isformed on an opposite side of the arrangement region of said switch andsaid voltage line segment from the circuit block, a first control linesegment to which a control signal is input via a control line isdisposed on an input side of said buffer circuit, and a second controlline segment electrically connected to a control node of said switch isdisposed on an output side of said buffer circuit.